> Even if you used 4 numbers for an alphanumeric password, it’s still much more secure than a 6 digit pin.
Unfortunately, that's not the case:
If you trust the secure enclave (for the device unlock scenario) or Apple's HSMs (for the key escrow scenario), a 6-digit PIN is just as secure as a 4-character alphanumeric password. In both cases, you get 10 invalid attempts before your data is wiped, and the odds are negligibly small in either case (10/10*6 vs. 10/62*4).
If you don't, i.e. you are concerned your adversary can somehow perform a brute-force attack, you need way more than four alphanumeric characters.
Unfortunately, that's not the case:
If you trust the secure enclave (for the device unlock scenario) or Apple's HSMs (for the key escrow scenario), a 6-digit PIN is just as secure as a 4-character alphanumeric password. In both cases, you get 10 invalid attempts before your data is wiped, and the odds are negligibly small in either case (10/10*6 vs. 10/62*4).
If you don't, i.e. you are concerned your adversary can somehow perform a brute-force attack, you need way more than four alphanumeric characters.